The invention relates generally to lithography, and, more particularly, to deep ultraviolet and extreme ultraviolet lithographic exposure systems.
In microchip manufacturing, microlithography has been at the forefront of manufacturing techniques. Lithography is also generally the main factor that drives microchip technology and speed. As microchips require finer pattern structures in order to increase speed, the resolution of lithographic manufacturing techniques must keep pace. Many improvements in the lithographic process have involved utilizing illumination sources having shorter wavelengths to pattern the circuitry onto a substrate wafer. The shorter the wavelength of the illumination source, typically the better the resolution and quality of the circuit lines patterned onto the substrate wafer.
One such lithographic process is deep ultraviolet lithography which is currently used for most microchip manufacture. Depending of the specific lithography process used, the deep ultraviolet lithography process typically operates at a wavelength of 248 nanometers and utilizes lenses to illuminate a design pattern (e.g., a mask or reticle) allowing throughput light to form images on the substrate wafer. Other deep ultraviolet lithographic techniques are being developed with shorter wavelengths of 193 and 157 nanometers.
Generally, the deep ultraviolet lithography process is currently limited to forming circuits with geometries (also known as lines) of xcx9c100 nanometers. Future generations of deep ultraviolet lithography are expected to be limited to forming circuits with some geometries of 50 nanometers. In order to improve upon this, another lithographic process currently being developed is extreme ultraviolet lithography (EUVL). The EULV process utilizes an illumination wavelength approaching that of x-rays, also termed xe2x80x9csoft x-raysxe2x80x9d, generally in the range of 8-13 nanometers, and more particularly at 13 nanometers. EULV is capable of creating circuits having a 30 nanometer resolution.
Resolution enhancement techniques (RET) have also been utilized to increase the resolution, depth of focus, contrast and quality of the circuitry lines. One such technique involves off-axis illumination of the illumination source onto the design plate. As the size of features on the design plates approach or drop below the wavelength of the exposure illumination, diffraction effects become more pronounced. This can cause the first order of the diffracted image to fall outside the projection lens, thereby causing imaging problems because both the zero and first diffraction orders must fall on the projection lens in order to properly resolve the image from the design plate. Off-axis illumination involves symmetrically illuminating the design plate from more than one direction off the optical axis of the lithography system. The zero and a first diffraction orders from each illumination point reach the lens with the symmetrical arrangement compensating for any shift in the image. Alternatively, imaging errors in the horizontal-to-vertical line performance or aberrations in the projection lens may require asymmetrical illumination in order to compensate for the error, including adjusting the illumination balance throughout the illumination pattern (e.g., causing one pole to be of higher intensity than another) and/or adjusting the shape of the illumination pattern (e.g., the horizontal of the of the illumination pattern is elongated to be larger than the vertical of the illumination pattern).
Various illumination modes may be utilized, including annular illumination or multipole illumination. Annular illumination may include a single ring, concentric rings, etc. While dipole and quadrapole illumination modes are commonly used, multipole illumination modes may include illumination patterns having any number of poles, including octapole configurations.
Many proposals for creating these illumination modes have included using multiple illumination sources, refraction, microlens arrays, spatial filters, illumination blockers, etc. Multiple illumination sources increases the size, cost and complexity of the lithography system. Refraction becomes impractical as the wavelength becomes smaller, given the transmission properties of optical lenses degrade, and in fact absorb much of the light. For example, at 157 nanometers, most materials are opaque to the illumination and at EUVL wavelengths the illumination is almost totally absorbed by refractive optics. Microlens arrays also tend to degrade at many of the shorter wavelengths described above. Even in deep ultraviolet wavelengths, these effects can occur along with thermally induced distortions, chromatic aberrations, birefringence, etc. These effects are even more pronounced at EUVL wavelengths. Blocking designs have been found to be inefficient. Reflective optics, on the other hand, work well with deep ultraviolet lithography illumination and EUVL.